Conventional spring-loaded contact probes generally include a moveable plunger, a barrel having an open end for containing an enlarged diameter section or bearing of the plunger, and a spring for biasing the travel of the plunger in the barrel. The plunger bearing slideably engages the inner surface of the barrel. The enlarged bearing section is retained in the barrel by a crimp near the barrel's open end.
The plunger is commonly biased outwardly a selected distance by the spring and may be biased or depressed inwardly into the barrel, a selected distance, under force directed against the spring. Axial and side biasing of the plunger against the barrel prevents false opens or intermittent points of no contact between the plunger and the barrel. The plunger generally is solid and includes a head, or tip, for contacting electrical devices under test. The barrel may also include a tip opposite the barrel's open end.
The barrel, plunger and tip form an electrical interconnect between the electrical device under test and test equipment and, as such, are manufactured from an electrically conductive material. Typically, the probes are fitted in cavities formed through the thickness of a test plate or socket. Generally, a contact side of the electrical device to be tested, such as an integrated circuit, is brought in to pressure contact with the tips of the plungers protruding through one side of the test plate or test socket for maintaining spring pressure against the electrical device. A contact plate connected to the test equipment is brought to contact with the tips of the plungers protruding through the other side of the test plate or test socket. The test equipment transmits test signals to the contact plate from where they are transmitted through the test probe interconnects to the device being tested. After the electrical device has been tested, the pressure exerted by the spring probes is released and the device is removed from contact with the tip of each probe. In conventional systems, the pressure is released by moving the electrical device and probes away from one another, thereby allowing the plungers to be displaced outwardly away from the barrel under the force of the spring, until the enlarged diameter bearing the plunger engages the crimp of the barrel.
The process of making a conventional spring probe involves separately producing the compression spring, the barrel and the plunger. The compression spring is wound and heat treated to produce a spring of a precise size and of a controlled spring force. The plunger is typically turned on a lathe and heat treated. The barrels are also sometimes heat treated. The barrels can be formed in a lathe or by a deep draw process. All components may be subjected to a plating process to enhance conductivity. The spring probe components are assembled either manually or by an automated process. The assembly process for these probes is a multiple step process. Considering that probes are produced by the thousands, a reduction in the equipment and the steps required to produce the probes will result in substantial savings.
An important aspect of testing integrated circuit boards is that they are tested under high frequencies. As such, impedance matching is required between the test equipment and integrated circuit so as to avoid attenuation of the high frequency signals. Due to the numerous probes that are used in relatively small area in the socket, the spacing between probes is minimal making impedance matching infeasible. In such situations, in order to avoid attenuation of the high frequency signals, the length of the electrical interconnects formed by the probes must be kept to a minimum. With current probes, when the interconnect length is minimized, so is the spring length and thus spring volume.
A spring's operating life, as well as the force applied by a spring are proportional to the spring volume. Consequently, the spring volume requirements for a given spring operating life and required spring force are in contrast with the short spring length requirements for avoiding the attenuation of the high frequency signals. Since the diameter of the spring is limited by the diameter of the barrel which is limited by the diameter of the cavities in the test sockets, the only way to increase the spring volume for increasing the spring operating life, as well as the spring force, is to increase the overall barrel length. Doing so, however, results in a probe having an electrical interconnect of increased length resulting in the undesirable attenuation of the high frequency signals.
An alternative type of conventional probe consists of two contact tips separated by a spring. Each contact tip is attached to a spring end. This type of probe relies on the walls of the test plate or socket cavity into which it is inserted for lateral support. The electrical path provided by this type of probe spirals down the spring wire between the two contact tips. Consequently, this probe has a relatively long electrical interconnect length which may result in attenuation of the high frequency signals when testing integrated circuits.
Thus, it is desirable to reduce the electrical interconnect length of a probe without reducing the spring volume. In addition, it is desirable to increase the spring volume without decreasing the spring compliance or increasing the electrical interconnect length. Moreover, a probe is desirable that can be easily manufactured and assembled.